textproduct: Rapid City

KEY MESSAGES

- Upper ridge will dominate the weather through next week.

- Thunderstorm areal coverage will increase tonight and hang about into next week, although in the hit or miss variety.

- Isolated strong/severe thunderstorms may occur during the later afternoon/evening hours today through Tuesday.

- Temperatures will average 4-8F above normal.

DISCUSSION

(This Evening Through Saturday) Issued at 1236 PM MDT Sat Aug 22 2026

18z surface analysis had weak high over the northern Plains into the middle MO river valley. Weak frontal boundary behind it from central MT into eastern CO. Patch of low stratus over western NE has been pushed northwestward on southeasterly winds along boundary leading to upper 50s Td/s across parts of northeastern WY. Water vapour matinee depicted shortwave ridge over the CWA, but shortwave trough hot on it's heels over eastern ID/western WY. Lift ahead of wave leading to some weak radar returns over northeastern WY, but nothing below 12kft per KGCC observation. Main concern this forecast is convection.

Later this afternoon/evening, around 1500J/kg MLCAPE pools along frontal boundary with weak MLCIN. Shortwave overtops, which will assist convective initiation. Perhaps 20-30kt effective shear will be present, enough to support isolated strong/severe thunderstorms where SPC Day 1 Marginal in place. Storms will slowly move eastward across the CWA tonight, but in weaker fashion over western SD given less favorable environment. Temperatures will be near guidance.

Sunday, aforementioned frontal boundary ends up over the western SD plains, influenced by tonight's convection/outflow. The further east it goes, the less the effect on the CWA. Right now, HREF suggests it'll settle over the eastern quarter of the CWA. 2KJ/kg CAPE pools along it with 30-40kts of 0-6km shear sufficient for strong/severe thunderstorms. Some CAM guidance suggests a supercell or two dropping south along the UNR/ABR border area late in the day. Temperatures will be near guidance.

For Monday/Tuesday, disturbances rotating around the upper ridge will support additional chances for strong/severe convection given forecast shear/buoyancy profiles. The rest of next week looks less active as upper ridge/height rises more dominant.

AVIATION

(For the 00Z TAFS Through 00Z Sunday Evening) Issued At 548 PM MDT Sat Aug 22 2026

Breezy southeast winds will continue into tonight, especially across northeast Wyoming, relaxing with time. Confidence remains low in thunderstorm potential and timing/evolution, as upper-level support remains off to the west and widespread cloud cover has kept things more stable this afternoon than anticipated. Best chance of storms remains across northeast Wyoming this evening, and have retained the PROB30 for KGCC through 04z/10pm. Some form of lingering showers or storms may be able to transition east across South Dakota overnight, with perhaps the best chances along and south of I-90. Brief vsby drops to MVFR along with gusty winds are the primary thunderstorm threats. Another round of isolated thunderstorms is likely Sunday afternoon, but there is currently significant disagreement among the models on pinning down when and where. The far eastern plains well east of Rapid City may be the best corridor to watch, but some CAMs also fire a cell or two off the Black Hills, with perhaps a cell or two arriving in northeast Wyoming as well by later in the evening. For now, confidence is too low to introduce any PROB30s, and will continue to monitor future updates in high-res guidance to see if better consensus forms.

UNR WATCHES/WARNINGS/ADVISORIES

SD...None. WY...None.